Accessory carpal bones

Accessory carpal bones are supernumerary ossicles of the wrist—small, inconstant bones that occur in addition to the eight normal carpal bones. They arise either from persistent or separate (accessory) ossification centers that fail to fuse with a neighboring carpal bone during development, or, in some locations, as sesamoid bones within tendons. They are congenital variants that are usually asymptomatic and clinically insignificant, but they are important because they can be mistaken on imaging for avulsion fractures, loose bodies, osteoid osteoma niduses, or other pathology.

Key features:

• Number and terminology. Approximately 30 distinct accessory carpal elements have been described in the human wrist, though only the os centrale is included in the official Terminologia Anatomica. The os centrale carpi is the most common accessory carpal bone; related "central" ossicles include the os centrale volare (praescaphoid), os epilunatum, and os hypolunatum.

• Origin. Most represent failure of fusion of an accessory ossification center (e.g., the "primordial carpal unit") with an adjacent carpal or metacarpal bone; others are true intra-articular ossicles or tendinous sesamoids.

• Clinical relevance. Though generally incidental, they can occasionally become symptomatic—causing painful clicking, crepitus, or pain—and rare complications such as osteonecrosis, coalition-associated deformity, or bone bruising simulating a scaphoid fracture have been reported. The os styloideum ("ninth carpal bone") at the carpometacarpal region, with a prevalence of ~1–4%, is a related accessory ossicle associated with carpal boss.

Note that in veterinary anatomy, the term "accessory carpal bone" (os carpi accessorium) refers instead to a specific, normal bone of the proximal carpal row in quadrupeds (analogous to the human pisiform), rather than a supernumerary variant.